Xhosacetus hendeysi Bianucci, Lambert & Post, 2007

Lambert, Olivier, Muizon, Christian de, Duhamel, Guy & Plicht, Johannes Van Der, 2018, Neogene and Quaternary fossil remains of beaked whales (Cetacea, Odontoceti, Ziphiidae) from deep-sea deposits off Crozet and Kerguelen islands, Southern Ocean, Geodiversitas 40 (6), pp. 135-160 : 153-155

Xhosacetus hendeysi Bianucci, Lambert & Post, 2007

REFERRED SPECIMEN AND LOCALITY. — Partial cranium MNHN.F.COI10 including rostrum, facial region, and vertex (Fig. 12); geographic coordinates 48°00’42.0”S, 65°34’09.6”E (255 km NWW to Kerguelen Islands) ; depth 1145 m ( Fig. 1 ).

BRIEF DESCRIPTION AND COMPARISON

The apex of the rostrum of this well-preserved specimen is incomplete. The mesorostral groove is completely filled with the vomer. The dorsal surface of the latter becomes progressively keeled from mid-length of the rostrum in the posterior direction and a longitudinal sulcus marks the right and left vomer-premaxilla contacts. A median suture is visible on the posterior half of the vomer in the mesorostral groove. The vomer-presphenoid suture is transversely directed, located at a level posterior to the premaxillary foramen. Deep longitudinal openings on the lateral surfaces of the rostrum most likely result from partial wear of superior alveolar canals. The shallow groove appearing ventral to these openings in the anterior half of the rostrum is probably a vestigial alveolar groove. The lateral margin of the rostrum raises posterodorsolaterally, forming a robust ridge that laterally defines a medial depressed area made of the premaxillae and vomer. No distinct prominential notch being present, the ridge joins the elevated, dome-like rostral maxillary crest, the latter being posterior and slightly medial to the widely open and relatively deep antorbital notch. The right rostral maxillary crest is transversely narrower than the left, slightly higher, and extends more posteriorly. A large dorsal infraorbital foramen is anteromedial to each rostral maxillary crest.

The premaxillary foramen is located at the level of the antorbital notch, and the dorsal surface of anteriormost portion of the premaxillary sac fossa slopes anteromedially. At its maximum width, the premaxillary sac fossa is transversely concave. The relatively short ascending process of premaxilla displays an abrupt elevation, with its anterior surface becoming roughly vertical in its upper part; the left process is slightly less erected. Right and left ascending processes display a strong transverse constriction as seen in anterior view. Each premaxillary crest is directed laterally and slightly posterolaterally, with a curve more pronounced on the slightly longer right crest. The nasals occupy a large area on the vertex, with the right nasal longer and wider than the left. The anteromedial tip of the joined nasals reaches the same anteroposterior level as the anterior margin of the premaxillary crests. The anterodorsal margin of each nasal is excavated by a wide, subvertical groove, best seen in dorsal view. The dorsal surface of the nasals displays a slight medial depression and anteromedial slope. The contribution of each nasal to the corresponding premaxillary crest is minor.

In addition to similar dimensions ( Table 9), this specimen shares many features with the holotype of Xhosacetus hendeysi , including the size and shape of the nasals on the vertex, the morphology of the premaxillae in the facial region, the developed rostral maxillary crests (erroneously named maxillary crests and premaxillary crests in different

maxilla-palatine suture

FIG. 12. — Partial cranium of Xhosacetus hendeysi (MNHN.F.COI10): A, dorsal view; B, same view with interpretive line drawing; C, detail of the vertex in dorsal view; D, anterior to slightly anterodorsal view; E, right lateral view. Scale bar: 100 mm.

A

views of the holotype in Bianucci et al. 2007: fig. 24), and the sulci along the vomer in the mesorostral groove. The main difference concerns the antorbital region; as in Pterocetus benguelae , the specimen MNHN.F.COI10 lacks a prominential notch, and its antorbital notch is more widely open and relatively deeper than in the holotype of X. hendeysi . Furthermore, the anteromedial slope of the nasals is somewhat more pronounced and the lateral margins of the nasals diverge slightly more anteriorly as compared to the holotype, two features in an intermediary state between the holotype and P. benguelae . However, the larger P. benguelae differs markedly in the more anterior position of the premaxillary foramen, the transversely broader antorbital region, and the lower rostral maxillary crest.

Figures

FIG. 1. — Locality maps: A, schematic map providing the position of Crozet and Kerguelen islands in the Southern Ocean; B, schematic map of the Crozet and Kerguelen islands area, including the main localities where fossil ziphiid specimens («) described in this work were found via longline fishing,as well as two ODP sites (l). Light grey shading for areas with a water depth lower than 1000 m; dark grey shading for islands and continents.